1. Field of the Invention
The present invention relates to a method for growing a single crystal of cubic boron nitride semiconductor, a method for forming a p-n junction of cubic boron nitride semiconductor and a light emitting element, particularly a light emitting element composed essentially of cubic boron nitride crystal and capable of emitting light within a wide range of from infrared to ultraviolet region.
2. Discussion of Background
Heretofore, silicon and gallium-arsenide have been widely used as semiconductor material. However, they have a drawback that they are hardly useful at high temperatures. For instance, a silicon semiconductor can not be used at a temperature of 150.degree. C. or higher, and a gallium-arsenide semiconductor can not be used at 250.degree. C. or higher.
On the other hand, cubic boron nitride is expected as a semiconductor useful at high temperatures. However, it is a substance synthesized under high pressure and high temperature conditions. Accordingly the process for its production is very much restricted. Namely, cubic boron nitride semiconductors have so far been prepared under high temperature and high pressure conditions by using a solvent containing a substance capable of providing semiconductor properties (hereinafter referred to as a "dopant") and hexagonal boron nitride, as starting materials and by utilizing the solubility of both materials as the driving force. In this method, crystals form in the vicinity of the starting material by spontaneous formation of seed crystals. Accordingly, the size of a crystal is restricted by the adjacent crystals and used to be limited to 1 mm at the maximum and usually to a level of not more than 0.5 mm. Therefore, cubic boron nitride semiconductors have not been practically developed, although they are expected to have excellent properties as semiconductors useful at high temperatures.
Heretofore, a p-n junction, a p-p junction and a n-n junction of a silicon or gallium-arsenide semiconductor have been widely used as electronic elements. However, such a p-n junction has a drawback that it is hardly operable at high temperatures. For instance, the operation for e.g. rectification tends to be difficult at a temperature of about 150.degree. C. in the case of a p-n junction of a silicon impurity semiconductor and at a temperature of about 250.degree. C. in the case of a gallium-arsenide impurity semiconductor.
On the other hand, cubic boron nitride is an insulator having a band gap as wide as diamond, and its impurity semiconductor doped with an impurity is expected to be a semiconductor useful at high temperatures. However, both materials are producible under very high pressure and high temperature conditions, and the formation of their p-n junctions required for devices has been extremely restricted and has not been practically accomplished.
Heretofore, gallium arsenide and gallium phosphide have been known and practically developed as materials for light emitting elements having a p-n junction. However, the emissions of these light emitting elements have been restricted within a range of from infrared to green. Silicon carbide and zinc selenide have been developed as materials for blue-emitting elements. However, with silicon carbide, it is difficult to obtain a crystal having good quality, and with zinc selenide, it is hardly possible to obtain a p-type crystal. Therefore, these materials have not yet been practically developed. Further, with gallium nitride or zinc sulfide having an energy gap at the ultraviolet region, it is difficult to obtain a p-type crystal. There has been no ultraviolet-emitting p-n junction light emitting element.